Resonance labels are more particularly used in commerce for marking goods and to prevent theft. The resonance labels fixed to the goods must be deactivated at the checkout, so that no false theft alarm is triggered on leaving the sales area. Consequently this deactivation must be carried out in such a way that it is permanent and reliable. The deactivation process must not be problematical and must instead be simple and reliable to perform.
Numerous proposals have been made for carrying out the deactivation of resonance labels, in which the resonance characteristics of the resonant circuit of the label are destroyed or changed.
Thus, U.S. Pat. No. 3,624,631 describes a deactivation, in which a fuse connected in series in the resonant circuit is blown, which takes place with the aid of a wobbulator. To comply with the postal regulations alone the energy radiated must not exceed a given level. Therefore the fuse must be as small as possible, so that the Q factor and consequently the detection possibility of the resonant circuit are reduced. It must also be made from a material blowing at the energy level complying with the regulations. This represents a relatively expensive and complicated manufacture of such resonance labels, particularly if it is borne in mind that account must also be taken of the thermal conductivity of the material surrounding the fuse.
U.S. Pat. No. 3,810,147 describes a multiple resonance label, which has different frequencies for detection and deactivation. Here again deactivation takes place by the blowing of a fuse, which is located in the deactivation resonant circuit. This eliminates possible false alarms, which could occur if the detection and deactivation frequency were identical. The dimensioning of the deactivation circuit containing the fuse takes place under the standpoint that the series impedance of the induction coil and the capacitor are to be kept as low as possible, so that most of the voltage drop is available for blowing the fuse. However, this means that the induction coil must be as small as possible and the capacitor as large as possible. However, the capacitor size leads to an undesired cost increase during manufacture and also to an impracticable increase in the size of the resonance label.
A fundamentally different possibility for deactivating a resonance label is based on the fact, that with a corresponding high potential, a breakdown takes place through the dielectric located between the two conductor circuits on either side of the resonance label. To keep the potential required for deactivation as low as possible, e.g. the dielectric layer is kept particularly thin.
U.S. Pat. No. 4,567,473 describes a resonance label having a notch in the dielectric between the capacitor plates. Deactivation takes place at or close to the resonant frequency with adequate energy, so that a breakdown takes place through the dielectric at the point defined by the notch. By means of an arc discharge and subsequent evaporation processes or plasma deposition, metal should be deposited along the breakdown path, so that a permanent short-circuit path is formed and as a result the resonance characteristics of this resonant circuit are destroyed. However, the manufacture of a precisely defined notch in a thin dielectric layer is relatively complicated and costly. It has therefore been proposed to replace this by moving the two capacitor plates towards one another by pressure at certain points and therefore reduce the dielectric thickness between the plates. However, once again difficulties have occurred during manufacture, particularly as a result of the small tolerances required. Even minor thickness fluctuations and material impurities of the dielectric often do not make it possible to obtain the desired, clearly defined thickness reduction.
EP-A1-0285559 describes another variant, according to which there is provided at least one hole through the dielectric between the capacitor plates. As a result a locally bounded, but clearly defined inhomogeneity is incorporated at which the breakdown between the capacitor plates can take place. As opposed to the aforementioned U.S. Pat. No. 4,567,473, it is possible to much better control the necessary geometry during manufacture, because when making a hole no thickness tolerances with respect to the dielectric have to be taken into account. However, all the above-described deactivation variants based on any type of reducing the dielectric thickness (a further variant being described in U.S. Pat. No. 4,689,636) additionally suffer from the disadvantage that at these thickness-reduced points the resonance label is weakened and therefore may not fulfill its function, e.g. in the case of bending stress.
DE-A1-3732825 and DE-A1-3826480 describe resonance labels, in which in each case a conductor coil is covered by a deactivation conductor, an insulating layer being placed between the coil and the conductor. This insulating layer is made electrically conductive in the case of an energy signal with an appropriately chosen energy. As a result the resonance label is deactivated. These resonance labels have several, namely at least two clearly defined breakdown points. As in this deactivation process possibly only part of the induction coil fails, this can lead to a frequency shift and therefore to the triggering of false alarms.
Thus, in various ways a number of variants have been tried out and used, in order to reliably and permanently deactivate a resonance label. The clearly contradictory requirements of reliability and durability of the deactivation on the one hand and of less expensive and more easily controllable manufacture on the other have as yet always made compromises necessary.